this site pretty much says it all:
http://talkorigins.org/faqs/faq-noahs-ark.html
an excerpt from there:
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Where did the Flood water come from, and where did it go? Several people have proposed answers to these questions, but none which consider all the implications of their models. A few of the commonly cited models are addressed below.
Vapor canopy. This model, proposed by Whitcomb & Morris and others, proposes that much of the Flood water was suspended overhead until the 40 days of rain which caused the Flood. The following objections are covered in more detail by Brown.
* How was the water suspended, and what caused it to fall all at once when it did?
* If a canopy holding the equivalent to more than 40 feet of water were part of the atmosphere, it would raise the atmospheric pressure accordingly, raising oxygen and nitrogen levels to toxic levels.
* If the canopy began as vapor, any water from it would be superheated. This scenario essentially starts with most of the Flood waters boiled off. Noah and company would be poached. If the water began as ice in orbit, the gravitational potential energy would likewise raise the temperature past boiling.
* A canopy of any significant thickness would have blocked a great deal of light, lowering the temperature of the earth greatly before the Flood.
* Any water above the ozone layer would not be shielded from ultraviolet light, and the light would break apart the water molecules.
Hydroplate. Walt Brown's model proposes that the Flood waters came from a layer of water about ten miles underground, which was released by a catastrophic rupture of the earth's crust, shot above the atmosphere, and fell as rain.
* How was the water contained? Rock, at least the rock which makes up the earth's crust, doesn't float. The water would have been forced to the surface long before Noah's time, or Adam's time for that matter.
* Even a mile deep, the earth is boiling hot, and thus the reservoir of water would be superheated. Further heat would be added by the energy of the water falling from above the atmosphere. As with the vapor canopy model, Noah would have been poached.
* Where is the evidence? The escaping waters would have eroded the sides of the fissures, producing poorly sorted basaltic erosional deposits. These would be concentrated mainly near the fissures, but some would be shot thousands of miles along with the water. (Noah would have had to worry about falling rocks along with the rain.) Such deposits would be quite noticeable but have never been seen.
Comet. Kent Hovind proposed that the Flood water came from a comet which broke up and fell on the earth. Again, this has the problem of the heat from the gravitational potential energy. The water would be steam by the time it reached the surface of the earth.
Runaway subduction. John Baumgardner created the runaway subduction model, which proposes that the pre-Flood lithosphere (ocean floor), being denser than the underlying mantle, began sinking. The heat released in the process decreased the viscosity of the mantle, so the process accelerated catastrophically. All the original lithosphere became subducted; the rising magma which replaced it raised the ocean floor, causing sea levels to rise and boiling off enough of the ocean to cause 150 days of rain. When it cooled, the ocean floor lowered again, and the Flood waters receded. Sedimentary mountains such as the Sierras and Andes rose after the Flood by isostatic rebound. [Baumgardner, 1990a; Austin et al., 1994]
* The main difficulty of this theory is that it admittedly doesn't work without miracles. [Baumgardner, 1990a, 1990b] The thermal diffusivity of the earth, for example, would have to increase 10,000 fold to get the subduction rates proposed [Matsumura, 1997], and miracles are also necessary to cool the new ocean floor and to raise sedimentary mountains in months rather than in the millions of years it would ordinarily take.
* Baumgardner estimates a release of 1028 joules from the subduction process. This is more than enough to boil off all the oceans. In addition, Baumgardner postulates that the mantle was much hotter before the Flood (giving it greater viscosity); that heat would have to go somewhere, too.
* Cenozoic sediments are post-Flood according to this model. Yet fossils from Cenozoic sediments alone show a 65-million-year record of evolution, including a great deal of the diversification of mammals and angiosperms. [Carroll, 1997, chpts. 5, 6, & 13]
* Subduction on the scale Baumgardner proposes would have produced very much more vulcanism around plate boundaries than we see. [Matsumura, 1997]
New ocean basins. Most flood models (including those above, possibly excepting Hovind's) deal with the water after the flood by proposing that it became our present oceans. The earth's terrain, according to this model, was much, much flatter during the Flood, and through cataclysms, the mountains were pushed up and the ocean basins lowered. (Brown proposes that the cataclysms were caused by the crust sliding around on a cushion of water; Whitcomb & Morris don't give a cause.)
* How could such a change be effected? To change the density and/or temperature of at least a quarter of the earth's crust fast enough to raise and lower the ocean floor in a matter of months would require mechanisms beyond any proposed in any of the flood models.
* Why are most sediments on high ground? Most sediments are carried until the water slows down or stops. If the water stopped in the oceans, we should expect more sediments there. Baumgardner's own modeling shows that, during the Flood, currents would be faster over continents than over ocean basins [Baumgardner, 1994], so sediments should, on the whole, be removed from continents and deposited in ocean basins. Yet sediments on the ocean basin average 0.6 km thick, while on continents (including continental shelves), they average 2.6 km thick. [Poldervaart, 1955]
* Where's the evidence? The water draining from the continents would have produced tremendous torrents. There is evidence of similar flooding in the Scablands of Washington state (from the draining of a lake after the breaking of an ice dam) and on the far western floor of the Mediterranean Sea (from the ocean breaking through the Straits of Gibralter). Why is such evidence not found worldwide?
* How did the ark survive the process? Such a wholesale restructuring of the earth's topography, compressed into just a few months, would have produced tsunamis large enough to circle the earth. The aftershocks alone would have been devastating for years afterwards.
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